1. Field of the Invention
The present invention relates to a structure of a vibration type drive unit such as an ultrasonic motor and the like and, in particular, to an improvement of the structure of a vibrator whose vibration is excited by applying a signal to a piezoelectric element.
2. Related Background Art
There is a vibration type drive unit, such as an ultrasonic motor and the like, in which a piezoelectric element that is an electromechanical energy conversion element is fixed on an elastic body, for example, made of a metal and an alternating signal is applied to this piezoelectric element to generate a progressive wave on the surface of the elastic body so as to move a moving element (rotor) which is at a pressure contact with the surface of elastic body by the progressive wave.
FIGS. 12A, 12B, 13A and 13B illustrate examples of the formations of conventional vibration type drive units.
FIG. 12A is a perspective view of a vibrator and a moving element (rotor) which is partly broken away, and FIG. 12B is a cross-sectional view in the rotation axis direction of the vibration type drive unit.
The reference numeral 101 denotes an annular elastic body made of a metal, and a piezoelectric element 102 is fixed on the bottom surface thereof. Grooves for enlarging vibration displacements are formed on the opposite surface of the piezoelectric element to the elastic body 101. Friction members 103 are arranged on the heads of protrusions formed by the grooves, and a rotor 104 is at a pressure contact with the friction members 103. The vibrator is composed of the elastic body 101, the piezoelectric element 102, and the friction members 103.
A flange portion whose thickness is thinner than the other parts is formed in the inner diameter portion of the elastic body 101 to extend toward the center of the circle, and this flange portion is fixed on a base member 108 of the vibration type drive unit. By disposing the flange portion having a spring characteristic on the fixing portion of the elastic body 101 and the base member 108, the vibrator can be supported without preventing the progressive wave from being generated. A disc spring 106 for pushing the rotor 104 in a direction toward the vibrator is fixed on the rotor 104, and since this disc spring 106 is fixed to a rotary shaft 105, the rotor 104 and the rotary shaft 105 rotate as a unit. Bearings 107 for rotatably supporting the rotary shaft 105 are provided on the base member 108. By disposing the plurality of bearings 107 in the axial direction, sway of the rotary shaft 105 is prevented.
An electrode pattern capable of generating two standing waves which are shifted ¼ wavelengths mutually is formed on the piezoelectric element 102, and when these two standing waves are excited while the phases thereof are shifted 90 degrees on a time scale, a progressive wave occurs on the surface of the elastic body 102. The rotor 104 is rotated and moved as if it were pushed out by the progressive wave.
This vibration type drive unit has a strong static torque at the time electricity is not turned on and has a strong rotational torque at the time the unit is driven so that it is possible to rotate and move the rotor to a desired position with high accuracy.
However, the vibration type drive unit shown in FIGS. 12A and 12B is not suitable for being miniaturized so much due to its shape.
Thus, there is a rod type vibration type drive unit shown in FIGS. 13A and 13B in order to obtain a more miniaturized one than the annular type vibration type drive unit shown in FIGS. 12A and 12B. FIG. 13A is a perspective view of a vibrator, and FIG. 13B is a cross-sectional view in the rotation axis direction of the vibration type drive unit.
The reference numerals 201, 202 indicate metal blocks, and a piezoelectric element 203 is disposed therebetween. The metal blocks 201, 202 and the piezoelectric element 203 have through holes, and a thread portion is formed on the inner diameter portion of the metal block 201. A supporting member 204 of the vibrator is inserted from the metal block 202 side so that the screw threads formed on the supporting member 204 are engaged with the screw threads of the inner diameter portion of the metal block 201. By tightening the supporting member 204, the piezoelectric element 203 and the metal block 202 are pressed to be fixed between the metal block 201 and a flange of an end portion of the supporting member 204. A narrow portion whose diameter is narrow for increasing vibration displacements is formed on the elastic body 201. Screw threads are formed on the distal end portion of the supporting member 204, a fixing member 208 is engaged with this distal end portion to be fixed by a nut 209, and an output gear 206 is rotatably supported about this fixing member 208. A rotor 205 is engaged with the output gear 206 so that when the rotor 205 rotates, the output gear 206 rotates and moves as a unit. A pressure spring 207 is disposed between the rotor 205 and the output gear 206 to impart a pressing force toward the elastic body 201 to the rotor 206.
When an alternating signal is applied to the piezoelectric element 203, two bending vibrations whose amplitude directions are orthogonal or approximately orthogonal to the axial direction and whose amplitude directions are mutually orthogonal or approximately orthogonal are excited, and these two bending vibrations are synthesized so that such a rotational movement as the elastic body 201 draws a circle approximately about the supporting member 204 occurs. Due to this rotational movement the rotor 205 which is at the pressure contact with the surface of the vibrator (that is, the surface of the elastic body 201) rotationally moves as if it were pushed out.
The vibration type drive unit shown in FIGS. 13A and 13B has a shape simpler than that of the vibration type drive unit shown in FIGS. 12A and 12B, and it can realize miniaturization particularly in the radial direction of the rotor.
However, as seen in FIGS. 13A and 13B, since the vibrator is constructed in such a way that the piezoelectric element is sandwiched between two elastic bodies, miniaturization for the size in the longitudinal direction of the supporting member of the vibration type drive unit has not been achieved so much compared with the size in the radial direction of the rotor. In order to miniaturize the size in the longitudinal direction of the supporting member, when the vibrator is simply miniaturized, the natural frequency of the vibrator increases and the vibration displacements decrease. Due to the increase of the natural frequency, an element in a drive circuit becomes expensive. Accordingly, it is deemed that there is still room for improvement in miniaturization of the size in the longitudinal direction of the supporting member of the vibration type drive unit without increasing the natural frequency of the vibrator.